Vehicle occupant detection systems are useful in connection with air bags and other pyrotechnically deployed restraints as a means of judging whether, and how forcefully, to deploy the restraint. One fundamental parameter in this regard is the weight of the occupant, as weight may be used as a criterion to distinguish between an adult and an infant or small child.
Most prior weight estimation techniques involve installing a pressure sensitive element such as a variable resistance pad or a fluid filled bladder in or under a vehicle seat cushion, and utilizing the pressure measurement as an indication of occupant weight. See, for example, the U.S. Pat. Nos. 5,474,327, 5,987,370, 6,246,936, 6,101,436 and 6,490,936, assigned to the assignee of the present invention and incorporated by reference herein.
Alternatively, the occupant weight may be measured with one or more load cells that sense the forces (strain or pressure) that the seat applies to a bracket that supports the seat on the vehicle floor. See, for example, the Publication Nos. 41520, 41542, 41549 and 41559 from the November, 1998 issue of Research Disclosure. In most applications, the load cells utilize one or more strain gauge elements for sensing the vertical or z-axis force applied through the load cell due to occupant weight. One of the problems encountered with this approach is that strain gauge elements also tend to be sensitive to bending or cross-axis forces that are not functionally related to occupant weight. The bending or cross-axis forces can occur, for example, when the adjoining surfaces of the load cell and the sensor are mismatched, or when the seat applies a twisting force to the load cell. In any event, the presence of such twisting or cross-axis forces tends to corrupt the z-axis force measurement, leading to inaccurate occupant weight estimation. Accordingly, what is needed is a seat frame load cell that applies z-axis forces to a strain gauge sensor while minimizing the effects of forces that are not functionally related to occupant weight.